C h a p t e r 1 4
Mechanisms of Infectious Disease
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visible colony composed of millions of bacteria within
a few hours.
In nature, however, bacteria rarely exist as single
cells floating in an aqueous environment. Rather, bac-
teria prefer to stick to and colonize environmental sur-
faces, producing structured communities called
biofilms
(Fig. 14-6). The organization and structure of biofilms
permit access to available nutrients and elimination of
metabolic waste. Within the biofilm, individual organ-
isms use chemical signaling as a form of primitive inter-
cellular communication to represent the state of the
environment. These signals inform members of the com-
munity when sufficient nutrients are available for pro-
liferation or when environmental conditions warrant
dormancy or evacuation. Examples of biofilms abound
in nature and are found on the surfaces of aquatic
environments, in the human oral cavity, on indwelling
medical devices, and on human cells. One has only to
FIGURE 14-5.
The manner in which
microorganisms divide (arrows) can assist in
their identification. Photomicrographs of gram-
positive
(A)
Staphylococcus aureus, which divide
in clusters;
(B)
Streptococcus spp. bacteria,
which divide in chains; and
(C)
urethral exudate,
which is diagnostic of gonococcal urethritis
revealing intracellular (within polymorphonuclear
leukocytes) and extracellular Gonococci, which
divide in pairs. (From the Centers for Disease
Control and Prevention Public Health Images
Library. Nos. 2296, 2170, 4086. A courtesy of
Richard Facklam; C courtesy of Norman Jacobs.)
A
B
C
FIGURE 14-4.
Microscopic morphology of
bacteria demonstrating their variability in size and
shape:
(A)
colorized scanning electron micrograph
(SEM) of the spherical gram-positive
β
-hemolytic
group C Streptococcus sp.;
(B)
colorized SEM of
the rod-shaped gram-negative Escherichia coli
of the strain O157:H7; and
(C)
corkscrew-shaped
spirochete Borrelia burgdorferi (responsible for
causing Lyme disease) shown using darkfield
microscopy technique (magnified ×400). (From
the Centers for Disease Control and Prevention
Public Images Library. Nos. 10591, 10586, 10068,
6631. A and B courtesy of Janice Haney Carr.)
A
B
C
